1. Field of the Invention
The invention relates to laser interferometer methods and apparatus, and more particularly to laser interferometers useful in positioning wafers in a semiconductor production system with improved isolation of the interferometer laser beam from environmental influences.
2. The Prior Art
FIG. 1 shows in simplified, schematic, perspective view a typical prior-art arrangement for positioning a semiconductor wafer 100. Wafer 100 is mounted in a chuck on an XY stage 105 having a bed 110 movable by operation of a controllable positioning device 115 along the X direction and a bed 120 movable by operation of a controllable positioning device 125 along the Y direction. The positioning devices and beds are mounted on a rigid platform 130. In the arrangement shown, stage 105 is used to position wafer 100 relative to an optical image produced by a light source 135, a reticle 140 and a projection lens assembly 145 in a step-and-repeat wafer exposure apparatus. The position of wafer 100 in each of the X and Y directions is measured by a laser interferometer system having a laser source 150, a beam splitter 155, interferometers 160 and 165, and mirrors 170 and 175. A laser beam 180 split by beam splitter 155 is supplied to interferometers 160 and 165. Interferometer 160 in turn splits the beam into a reference laser beam (not shown) and a measurement laser beam 185 which is applied to mirror 170 for measuring position of wafer 100 in the X direction. Interferometer 165 splits the beam into a reference laser beam (not shown) and a measurement laser beam 190 which is applied to mirror 175 for measuring position of wafer 100 in the Y direction. The measured position is fed back to a stage control circuit (not shown) which controls positioning devices 115 and 125 to position the wafer in a closed loop control system.
Known systems of this general type have the stage, wafer and optical system surrounded by atmospheric air, though typically in an air-conditioned environment to provide control of dust and air temperature. Even so, air temperature varies enough over the length of the measurement laser beams, over short periods of time, to introduce a significant error in the wafer-position measurement. These variations can result, for example, from heat-producing components such as positioners 115 and 125, light source 135, and/or laser source 150.
FIG. 2 illustrates the effect of environmental variation in position measurement. Distance L between interferometer 160 and mirror 170 is to be measured using a measurement laser beam 185 aligned in the X direction. Rather than directly measuring distance, interferometer 160 measures an optical path length OPL which is related to distance L and to the index of refraction .eta. of the air through which beam 185 passes by the relationship EQU OPL=.eta.L
Any variation in the index of refraction directly affects OPL and, with it, the apparent position of wafer 100. In situations where a differential interferometer is used, some compensation is possible if the reference beam can be located in an environment that is comparable to the measurement beam. This can compensate large-scale environmental changes but does not adequately compensate localized variations. For example, it has been calculated that to measure the apparent distance L of a 420 mm path length to within 1 nm, temperature and pressure of the air through which the measurement laser beam passes must be maintained to within 0.002.degree. K. and 0.006 mm Hg, respectively. Such tolerances are not believed achievable with existing technology.
U.S. Pat. No. 4,814,625 describes a semiconductor wafer-exposure system similar to that of FIG. 1 in which air-conditioning devices are provided for blowing currents of air at a controlled temperature toward the stage along the measuring paths of the laser interferometer system in the X and Y directions. Such a system attempts to limit air temperature variations which can affect measurement accuracy. U.S. Pat. No. 5,141,318 describes a laser-interferometer measuring apparatus and method for positioning a wafer in a semiconductor production system. Temperature-controlled air is passed through a vent which blows a uniform, laminar flow of air over the length of the measurement laser beam.
In practice, the systems of these two patents have at least two major drawbacks. First, they are difficult to implement due to limited space in the vicinity of the stage in a real wafer-stepper apparatus. Second, the improvement in measurement accuracy is believed inadequate for the expected demands of next-generation semiconductor processes. While such arrangements can reduce to some degree the influence of air temperature and pressure variations on measurement accuracy, more effective isolation from environmental fluctuations is needed.
Electron-beam lithography systems are known in which a wafer is positioned using a stage, and in which the electron optics and the stage and wafer are all enclosed in a vacuum chamber. While a vacuum environment is required for operation of an electron beam system, it is impractical and even undesirable for a semiconductor production system. Construction and maintenance of a vacuum system large enough for a wafer-stepper system would be costly and complex. Vacuum pumps, seals, housing elements and the like would increase initial cost and take up costly production space. System maintenance would be hindered by the need to assure vacuum sealing during system operation and after each intervention. Air locks would be needed to avoid losing vacuum each time a new wafer is introduced, possibly limiting achievable production rates. The changed index of refraction would mandate a complete redesign of the complex optical exposure system.
Despite its other disadvantages, the use of a vacuum environment offers the advantage of improved interferometer measurement accuracy due to reduced environmental influences on the measurement laser beams.